Method of selective photo-enhanced wet oxidation for nitride layer regrowth on substrates and associated structure

ABSTRACT

Various embodiments of the present disclosure pertain to selective photo-enhanced wet oxidation for nitride layer regrowth on substrates. In one aspect, a method may comprise: forming a first III-nitride layer with a first low bandgap energy on a first surface of a substrate; forming a second III-nitride layer with a first high bandgap energy on the first III-nitride layer; transforming portions of the first III-nitride layer into a plurality of III-oxide stripes by photo-enhanced wet oxidation; forming a plurality of III-nitride nanowires with a second low bandgap energy on the second III-nitride layer between the III-oxide stripes; and selectively transforming at least some of the III-nitride nanowires into III-oxide nanowires by selective photo-enhanced oxidation.

This is a continuation application of co-pending U.S. application Ser.No. 13/086,663, filed Apr. 14, 2011, the subject matter of which isincorporated herein by reference.

BACKGROUND

1. Technical Field

Various embodiments of the present disclosure relate to semiconductorlight-emitting devices and, more particularly, to a method of selectivephoto-enhanced wet oxidation for nitride layer regrowth on substrates.

2. Description of the Related Art

Light-emitting diodes (LEDs) are a type of semiconductor-based lightsource. An LED typically includes semiconducting materials doped withimpurities to create a p-n junction. The wavelength of the lightemitted, and thus its color, depends on the bandgap energy of thematerials forming the p-n junction. In silicon or germanium diodes, theelectrons and holes recombined by a non-radiative transition produce nooptical emission, because these are indirect bandgap materials. Thematerials used for the LED have a direct bandgap with energiescorresponding to near-infrared, visible or near-ultraviolet light. LEDsare usually built on an n-type substrate with an electrode attached tothe p-type layer deposited on its surface. LEDs built on p-typesubstrates, while less common, are available as well. Many commercialLEDs, especially GaN/InGaN, also use sapphire substrate.

Blue LEDs are based on the wide bandgap III-nitride materials such as,for example, GaN (gallium nitride) and InGaN (indium gallium nitride).Blue LEDs typically have an active region consisting of one or moreInGaN quantum wells sandwiched between thicker layers of GaN, calledcladding layers. By varying the relative InN-GaN molar fraction in theInGaN quantum wells, the light emission can be varied from violet toamber. AlGaN (aluminum gallium nitride) of varying AlN molar fractioncan be used to manufacture the cladding and quantum well layers forultraviolet LEDs, but these devices have not yet reached the level ofefficiency and technological maturity of the InGaN-GaN blue/greendevices. If the active quantum well layers are GaN, instead of alloyedInGaN or AlGaN, the device will emit near-ultraviolet light withwavelengths around 350-370 nm. Green LEDs manufactured from theInGaN-GaN system are far more efficient and brighter than green LEDsproduced with non-nitride material systems.

Advancements have been achieved in recent years to increase the externalquantum efficiency of LEDs. Among the various techniques, one knownapproach is to form a plurality of protrusions on a surface of a C plane(0001) sapphire substrate in a two-dimensionally repeated pattern, andthen epitaxially grow a number of GaN-based semiconductor layers on thesapphire substrate. The repeated pattern has a pitch greater than orequal to λ/4 and less than or equal to 20 μm, and side surfaces of theprotrusions have an inclined angle that is not less than 90° and notmore than 160°. Accordingly, the external quantum efficiency of LEDsthus formed is said to be increased as a result of an optical beamdiffraction mechanism due to the two-dimensional pattern of protrusionson the sapphire substrate.

With respect to forming a number of protrusions on a surface of asapphire substrate in a two-dimensionally repeated pattern, a number ofapproaches have been proposed but each is not without disadvantages. Forexample, one approach aligns a SiO₂ stripe mask parallel to the [11 2 0]direction of GaN to produce optically smooth layers on the inclinedsemi-polar {1 1 01} GaN facets. However, impurity migration from themask region (SiO₂) can cause serious material contamination issues tothe growth of nitride layers thereafter. Another approach aligns theSiO₂ stripe mask parallel to the [1 1 00] direction of GaN to produceoptically smooth layers on the inclined semi-polar {11 2 2} GaN facets.However, as with the aforementioned approach, impurity migration fromthe SiO₂ mask region can cause serious material contamination issues tothe growth of nitride layers thereafter.

On the other hand, V-notch grooves may be found in an LED structurecomprising adjacent layers of Al(Ga)N/In(Ga)N with high bandgap energyand low bandgap energy. Screw dislocations, which are undesirable, tendto exist at the bottom of the V-notch grooves. Etch pit density (EPD) isa measure for the quality of semiconductor wafers. An etch solution,such as molten KOH at 450° C., is applied on the surface of the waferwhere the etch rate is increased at dislocations, such as screwdislocations, of the crystal resulting in pits. To suppress furtherpropagation of screw dislocations, one approach utilizes aphotolithographic method with alignment procedures which requireadditional time and costs in operation and equipment. Other approachesrequire SiO₂ masking or dry etch. However, such techniques may result inundesirable impurity migration and/or damage to the crystal atomicstructure.

SUMMARY

Various embodiments of the present disclosure pertain to techniques offabrication of LED structures using selective photo-enhanced wetoxidation and suppression of propagation of screw dislocations in LEDstructures using selective photo-enhanced oxidation.

In one aspect, a method may comprise: forming a first III-nitride layerwith a first low bandgap energy on a first surface of a substrate;forming a second III-nitride layer with a first high bandgap energy onthe first III-nitride layer; transforming portions of the secondIII-nitride layer into a plurality of III-oxide stripes byphoto-enhanced wet oxidation; forming a plurality of III-nitridenanowires with a second low bandgap energy on the second III-nitridelayer between the III-oxide stripes; and selectively transforming atleast some of the III-nitride nanowires into III-oxide nanowires byselective photo-enhanced oxidation.

In one embodiment, the selective photo-enhanced wet oxidation maycomprise photo-enhanced wet oxidation with a photonic energy between thefirst high bandgap energy and the first low bandgap energy.

In one embodiment, the III-nitride nanowires may comprise In/GaNnanowires, and wherein the III-oxide nanowires comprise (In/Ga)2O3nanowires.

In one embodiment, the substrate may comprise a C plane (0001) sapphiresubstrate, wherein the second III-nitride layer comprises a layer ofGaN, and wherein the III-oxide stripes are aligned substantiallyparallel to a [11 2 0] or [1 1 00] direction of GaN.

In one embodiment, the method may further comprise: forming a thirdIII-nitride layer on the second III-nitride layer and over the III-oxidenanowires; and removing the III-oxide nanowires to form a plurality ofair gaps in the third III-nitride layer.

Forming a third III-nitride layer may comprise forming the thirdIII-nitride layer by lateral epitaxy of the III-nitride layer using ametal-organic chemical vapor deposition (MOCVD) process. Removing theIII-oxide nanowires may comprise dissolving the III-oxide nanowiresusing an acidic electrolyte or a basic electrolyte.

The method may further comprise filling the air gaps with a thermallyconductive material, the thermally conductive material comprising aplurality of nanoparticles, a plurality of compound particles, or acombination thereof.

Alternatively or additionally, the method may further comprise fillingthe air gaps with an electrically conductive material, the electricallyconductive material comprising a plurality of nanoparticles, a pluralityof compound particles, or a combination thereof.

In one embodiment, the method may further comprise forming a pluralityof III-nitride layers on the third III-nitride layer to form at leastone LED.

In another aspect, a method may comprise: forming a first III-nitridelayer with a first low bandgap energy on a first surface of a C plane(0001) sapphire substrate; forming a second III-nitride layer with afirst high bandgap energy on the first III-nitride layer; forming an LEDstructure over the second III-nitride layer; and forming, at least inpart by selective photo-enhanced oxidation, in the LED structure aplurality of air gaps that are parallel to one another and adjacent thesecond III-nitride layer.

In one embodiment, forming, at least in part by selective photo-enhancedoxidation, a plurality of air gaps that are parallel to one another andadjacent the second III-nitride layer may comprise: transformingportions of a surface of the second III-nitride layer into a pluralityof III-oxide stripes by photo-enhanced wet oxidation; forming aplurality of III-nitride nanowires with a second low bandgap energy onthe second III-nitride layer between the III-oxide stripes; selectivelytransforming at least some of the III-nitride nanowires into III-oxidenanowires by selective photo-enhanced oxidation; forming a thirdIII-nitride layer on the second III-nitride layer and over the III-oxidenanowires; and removing the III-oxide nanowires to form the plurality ofair gaps.

In one embodiment, the selective photo-enhanced wet oxidation maycomprise photo-enhanced wet oxidation with a photonic energy between thefirst high bandgap energy and the first low bandgap energy.

In one embodiment, removing the III-oxide nanowires may comprisedissolving the III-oxide nanowires using an acidic electrolyte or abasic electrolyte.

In one embodiment, the method may further comprise filling the air gapswith a thermally conductive material, the thermally conductive materialcomprising a plurality of nanoparticles, a plurality of compoundparticles, or a combination thereof. Alternatively or additionally, themethod may further comprise filling the air gaps with an electricallyconductive material, the thermally conductive material comprising aplurality of nanoparticles, a plurality of compound particles, or acombination thereof.

In one embodiment, the method may further comprise, after forming thesecond III-nitride layer on the first III-nitride layer and beforeforming the LED structure over the second III-nitride layer, forming asurface passivation layer in a V-notch groove that is in the secondIII-nitride layer or in a combination of the second III-nitride layerand the first III-nitride layer by selective photo-enhanced oxidation.

In one embodiment, the surface passivation layer may comprise aIII-oxide layer.

In one embodiment, forming the LED structure over the second III-nitridelayer may comprise forming a third III-nitride layer on the surfacepassivation layer and the second III-nitride layer by lateral epitaxy ofthe III-nitride layer using an MOCVD process.

In one aspect, a method of suppressing screw dislocation in an LEDstructure may comprise: forming a first III-nitride layer with a firstlow bandgap energy on a first surface of a substrate; forming a secondIII-nitride layer with a first high bandgap energy on the firstIII-nitride layer; and forming a surface passivation layer of III-oxidein a V-notch groove that is in the second III-nitride layer or in acombination of the second III-nitride layer and the first III-nitridelayer by selective photo-enhanced oxidation.

In one embodiment, the method may further comprise forming a thirdIII-nitride layer on the surface passivation layer and the secondIII-nitride layer by selective lateral epitaxy of the third III-nitridelayer using an MOCVD process.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description is described with reference to the accompanyingfigures. In the figures, the left-most digit(s) of a reference numberidentifies the figure in which the reference number first appears. Thesame reference numbers in different figures indicate similar oridentical items.

FIGS. 1A-1G are diagrams showing progression in fabrication of an LEDstructure in accordance with an embodiment to the present disclosure.

FIGS. 2A-2C are diagrams showing progression in suppression ofpropagation of screw dislocations in an LED structure in accordance withan embodiment to the present disclosure.

FIGS. 3A-3B are photos of examples in accordance with an embodiment tothe present disclosure.

FIG. 4 is a flowchart diagram showing a process of fabrication of an LEDstructure in accordance with an embodiment to the present disclosure.

FIG. 5 is a flowchart diagram showing a process of fabrication of an LEDstructure in accordance with another embodiment to the presentdisclosure.

FIG. 6 is a flowchart diagram showing a process of suppression of screwdislocations in an LED structure in accordance with an embodiment to thepresent disclosure.

FIGS. 7-11 are diagrams relating to a photo-enhanced chemical oxidation(etching) process in accordance with the present disclosure.

DETAILED DESCRIPTION Overview

Various embodiments of the present disclosure pertain to techniques ofselective photo-enhanced wet oxidation in fabrication of LED structuresand in suppression of propagation of screw dislocations in LEDstructures.

In a semiconductor structure where a layer of III-nitride with lowbandgap energy E_(g,Lo) and a layer of III-nitride with high bandgapenergy E_(g,Hi) are deposited one above the other, selectivephoto-enhanced wet oxidation with a photonic energy hy, whereE_(g,Lo)<hy<E_(g,Hi), can transform III-nitride compounds with E_(g,Lo)into III-oxide compounds. This technique can be utilized to formIII-oxide nanowires over the layers of III-nitride. Lateral expitaxy ofa new layer of III-nitride over the existing layers of III-nitride andIII-oxide nanowires can then be achieved by MOCVD. Afterwards, air gapscan be formed by dissolving the III-oxide nanowires using an acidicelectrolyte or a basic electrolyte. The air gaps provide crystal latticemomentum for optical diffraction to advantageously result in increasedexternal quantum efficiency similar to that achieved by the approach ofhaving a plurality of protrusions repeated in a two-dimensional patternon a surface of a C plane (0001) sapphire substrate. The air gaps alsopromote heat dissipation from the LED structure during operation.However, the proposed technique of the present disclosure advantageouslyremoves the source of material contamination due to the mask layer.

Suppression of propagation of screw dislocation at the bottom of V-notchgrooves in a layer of III-nitride with low bandgap energy in asemiconductor structure may be achieved by using selectivephoto-enhanced oxidation to form a surface passivation layer to fill inthe bottom portion of the V-notice grooves. Afterwards, a III-nitridelayer may be selectively grown by lateral epitaxy. This technique avoidsthe relatively higher costs and longer time associated with usingillumination alignment procedures. Issues such as undesirable impuritymigration and/or damage to the crystal atomic structure associated withSiO₂ masking or dry etch can also be avoided.

Referring to FIGS. 7-11, a photo-enhanced chemical oxidation/etchingprocess in accordance with the present disclosure will now be introducedas embodiments of the inventive techniques of the present disclosure, tobe described later, utilize one or more aspects of photo-enhancedchemical oxidation/etching.

Photo-Enhanced Chemical Oxidation (Etching)

A photo-enhanced chemical (PEC) oxidation/etching process is a kind ofphotochemistry process. In III-nitride materials, this process consistsof applying a ultraviolet (UV) irradiation onto a galvanic cell formedby immersing a III-nitride working electrode, a platinum (Pt) counterelectrode, and a reference electrode in an electrolyte. The resultantreaction is that the UV-excited hot carriers at theIII-nitride/electrolyte interface have excess energy to access the H+/H₂and OH—/O₂ redox levels in water and enhance the oxidative dissolutionof III-nitride specimen.

FIG. 7 shows an experimental setup. One or more samples are mounted on aTeflon base and clipped by a Pt washer. The Pt wire cathode and ammeterare used to monitor the reaction current flowing in the reaction cell. A253.7 nm mercury (Hg) line source is used to illuminate the whole samplesurface with UV light of intensity of about 20 mW/cm². Ultravioletillumination generates electron-hole pairs and enhances the oxidationand reduction reactions at the semiconductor surface. The solution canbe KOH, H₃PO₄, etc. for etching process and CH₃COOH/CH₃COONH₄ buffersolution for oxidation. It has been proposed that the PEC etching of GaNoccurs through oxidative decomposition, in which photo-generated holesassist in the oxidation and subsequent dissolution of the semiconductorinto aqueous acid or base solution. The oxidation reaction below isresponsible for the decomposition of GaN, where e⁻/h⁺ stands,separately, for photo-generated electron/hole.

Cathode: 2H2O+2e−→20H—+H2.   (1)

Anode: 2GaN+6OH—+6h+→Ga2O3+N2+3H2O.   (2)

Gallium oxide formed on the interface of semiconductor and electrolytecan be removed (etching) or preserved (oxidation) depending on thesolution in PEC process. The surface energy band diagrams for GaN ofdifferent doping types are shown in FIG. 8.

FIG. 8 illustrates the band bending of n- and p-type at thesemiconductor/electrolyte interface. Upward (n-GaN) and downward (p-GaN)band bending at the surface of the GaN occurs due to the types of dopantin the GaN films. For n-type GaN, the photo-generated holes can beeffectively accumulated near the semiconductor/electrolyte interfaceand, therefore, assists the reaction and oxidation of GaN. However, inp-type GaN the downward band bending accumulates the electrons near theinterface. The electron accumulation enhances the bond strength betweenGa-0 and results in not only the slowing down of the dissolution rate ofGa₂O₃ but also inhibiting further oxidation of GaN.

Photo-Enhanced Chemical Wet Oxidation Characteristics of GaN

There have been few reports on the oxidation or etching characteristicsof GaN or its related compounds from crystallographic aspects, e.g.,orientation effect on etching rates, shapes of pits and hillocks, anddissolution processes. To the inventors' knowledge, there has been noreport on the orientation effects of GaN wet chemical oxidation.

To this purpose, the inventors employed a 4 μm-thick n-type GaN epilayergrown on sapphire by metal-organic chemical vapor deposition (MOCVD).The substrate was lapped and polished to the thickness of about 90 μm topermit cleavage for the observation of etching profiles. A thin titaniummetal or photoresist strip of approximately 100 nm parallel to the [1120]_(GaN) direction was deposited on the sample by standardphotolithography and lift-off techniques. The electrochemical cell isshown in FIG. 7 with Ti or photoresist contact formed on the edge ofeach sample to serve as PEC working electrode and Pt as counterelectrode. A buffered acetic acid electrolyte (CH₃COOH/CH₃COONH₄) of pHvalue 6.4 was prepared to adjust the redox Fermi level with respect tothe band-edge of GaN. FIG. 9 displays an illustrative scanning electronmicrograph revealing the cleaved cross-sectional oxidation profile afterPEC oxidation for 2 hours and dehydrated at 350° C. for 1 hour.

It can be observed that volume expansion occurred in the conversion ofGaN to Ga₂O₃. Moreover, cracks on oxide indicate the release of straininside the oxide layer. It is interesting to note that the oxidationprofile reveals anisotropic reaction rate. From the underlying profilebeneath oxide, the oxidation rate along [000 1] is about 10 times fasterthan that along [1 100]_(GaN). The anisotropic property may result fromthe difference between both the surface energy and surface field of GaNof different crystallographic plane. In the present case, the PECoxidation process was enhanced by the internal field along [000 1]caused by surface band bending of GaN. As a result of the direction ofthe internal field (along c-axis), the non-polar plane of {1100}_(GaN and {)11 20}_(GaN) are less influenced and reveal sloweroxidation rate. Further investigation by repeated oxidation and oxideremoving by immersing in CH₃COOH solution on a 4 μm-thick GaN sample wascarried out with Ti or photoresist strip parallel to the [11 20]_(GaN)direction. The strip was 1.2 μm in width with GaN opening of the samesize. The GaN sample was first PEC oxidation for 2 hours followed byoxide removing, and successive oxidation for 2 hours was proceeded toreach a deep oxidation profile. As illustrated in FIG. 10, the resultantprofile exhibited pentagon shape bounded by {1 100}_(GaN) and {1103}_(GaN) with 60° inclined angle to the substrate normal.

With the crystallographic properties of PEC oxidation, various oxidationprofiles can be easily formed for the application such as the verticalfacet for laser, light emitting diodes, and microcavity fabrication.

Selective Photo-Enhanced Wet Oxidation (Etching)

The selective photo-enhanced wet oxidation (etching) process involvestriggering photo-enhanced wet oxidation with a photonic energy hy, whereE_(g,Lo)<hy<E_(g,Hi), can transform III-nitride compounds with a bandgapenergy of E_(g,Lo) into III-oxide compounds. For example, III-nitridessuch as (In/Ga)N can be transformed into III-oxide such as (In/Ga)₂O₃ byselective photo-enhanced wet oxidation. Consequently, the firstIII-nitride layer, having a low bandgap energy E_(g,Lo), is transformedinto a corresponding III-oxide layer, and the second III-nitride layer,having a high bandgap energy E_(g,Hi), maintains the originalcomposition. As the typical photo-enhanced wet oxidation (etching)process, III-oxide formed on the interface of semiconductor andelectrolyte can be removed (etching) or preserved (oxidation) dependingon the solution in this photochemistry process. FIG. 11 is the result ofselective photo-enhanced wet oxidation performed on AlGaN/GaNhetero-structure_with etched grooves. Selective transformation of GaNinto Ga₂O₃ can be done, with AlGaN remaining, without any metal maskbeing used.

IIIustrative Fabrication of an LED Structure

FIGS. 1A-1G are diagrams showing progression in fabrication of an LEDstructure in accordance with an embodiment to the present disclosure. Itis to be noted that, although none of the Figures is drawn to scale,FIGS. 1A-1B are similarly drawn to one scale while FIGS. 1C-1G aresimilarly drawn to another different scale.

FIG. 1A illustrates a substrate 110 having a first III-nitride layer 120deposited thereon, and a second III-nitride layer 130 deposited on thefirst III-nitride layer 120. In one embodiment, the substrate 110 is asapphire substrate. In one embodiment, the substrate 110 is a C plane(0001) sapphire substrate. In other embodiments, the substrate 110 maybe an A plane (11 2 0) sapphire substrate, an R plane (1102) sapphiresubstrate, or an M plane (1 1 00) sapphire substrate. The firstIII-nitride layer 120 and the second III-nitride layer 130 are layers ofnitride compounds including those from the boron family of the PeriodicTable, such as Al (aluminum), Ga (gallium), and In (indium) for example.In one embodiment, the first III-nitride layer 120 may comprise a layerof GaN, InN, AlGaN, or InGaN. In one embodiment, the second III-nitridelayer 130 may comprise a layer of GaN, InN, AlGaN, or InGaN. Between thefirst III-nitride layer 120 and the second III-nitride layer 130, thefirst III-nitride layer 120 has a low bandgap energy E_(g,Lo1) and thesecond III-nitride layer 130 has a high bandgap energy E_(g,Hi1) whereE_(g,Hi1)>E_(g,Lo1).

FIG. 1B illustrates that a plurality of III-oxide stripes 135(1)-135(M)are formed on the top surface of the second III-nitride layer 130. Morespecifically, the III-oxide stripes 135(1)-135(M) are formed by aphoto-enhanced wet oxidation process, such as one similar to thatdescribed above. Although a fixed number of the III-oxide stripes135(1)-135(M) are shown, the actual number of the III-oxide stripes135(1)-135(M) in various embodiments may be greater or smaller.Accordingly, M is a positive integer greater than 1. In one embodiment,when the second III-nitride layer 130 of a high bandgap energy comprisesAl/GaN, the III-oxide stripes 135(1)-135(M) comprise (Al/Ga)₂O₃,respectively. The III-oxide stripes 135(1)-135(M) are parallel to oneanother. In one embodiment, the III-oxide stripes 135(1)-135(M) alignparallel to the [11 2 0] or [1 1 00] direction of GaN.

FIG. 1C illustrates that a plurality of low bandgap energy III-nitridenanowires 140(1)-140(N) are grown on the second III-nitride layer 130between the III-oxide stripes 135(M-1) and 135(M). Although a fixednumber of the III-nitride nanowires 140(1)-140(N) are shown, the actualnumber of the III-nitride nanowires 140(1)-140(N) in various embodimentsmay be greater or smaller. Accordingly, N is a positive integer greaterthan 1. In one embodiment, the III-nitride nanowires 140(1)-140(N) havelow bandgap energy E_(g,Lo2), which may be the same as or different thanE_(g,Lo1). Given that the III-nitride nanowires 140(1)-140(N) are grownbetween the III-oxide stripes 135(M-1) and 135(M), the III-nitridenanowires 140(1)-140(N) are also parallel to one another and parallel tothe [11 2 0] or [1 1 00] direction of GaN.

FIG. 1D illustrates the results of a selective photo-enhanced oxidationprocess, such as one similar to that described above, that selectivelytransforms at least some of the III-nitride nanowires 140(1)-140(N) intoIII-oxide nanowires 145(1)-145(N). More specifically, selectivephoto-enhanced wet oxidation with a photonic energy hy, whereE_(g,Lo)<hy<E_(g,Hi), can transform III-nitride compounds with E_(g,Lo)into III-oxide compounds. For example, III-nitrides of a low bandgapenergy such as In/GaN can be transformed into (In/Ga)₂O₃ by selectivephoto-enhanced wet oxidation.

FIG. 1E illustrates that a third III-nitride layer 150 is formed on thesecond III-nitride layer 130 and over the III-oxide nanowires 135(M-1)and 135(M) (not shown in the far left end and far right end of FIG. 1E,respective). The third III-nitride layer 150 may be nucleated on surfacesites of the second III-nitride layer 130 not covered by the III-oxidenanowires 145(1)-145(N) nor by the III-oxide stripes 135(M-1) and135(M). Coalescence due to lateral epitaxial growth, or lateral epitaxy,occurs and results in a flat surface comprising the third III-nitridelayer 150. In one embodiment, the lateral epitaxy of the thirdIII-nitride layer 150 is done using an MOCVD process. Given thatIII-nitride compounds can nucleate on a C plane (0001) sapphiresubstrate but cannot grow on the surface of oxides, such as a III-oxidelayer, MOCVD can suitably be utilized for laterally epitaxial growth andformation of the third III-nitride layer 150.

FIG. 1F illustrates that an LED structure including one or more LEDs isformed on the third III-nitride layer 150. In one embodiment, as shownin FIG. 1F, a plurality of III-nitride layers, such as a fourthIII-nitride layer 160 and a fifth III-nitride layer 170, are depositedsequentially above the third III-nitride layer 150. For example, for aGaN-based LED structure, the third III-nitride layer 150 may comprisen-GaN, the fourth III-nitride layer 160 may comprise a multiple-quantumwell (MQW) emission layer, and the fifth III-nitride layer 170 maycomprise p-GaN. These III-nitride layers may comprise other compounds invarious implementations. In other embodiments, the number of III-nitridelayers deposited above the third III-nitride layer 150 may vary. Aplurality of LEDs may be made by cutting the LED structure shown in FIG.1F.

FIG. 1G illustrates the LED structure after the III-oxide nanowires145(1)-145(N) have been removed, leaving a corresponding plurality ofair gaps 180(1)-180(N). In one embodiment, given that III-oxidecompounds such as (In/Ga)₂O₃ dissolve in weak acidic or weak basicelectrolyte while III-nitride compounds do not, the III-oxide nanowires145(1)-145(N) are removed by immersing the LED structure in a weakacidic electrolyte, e.g., H₃PO₄, or a weak basic electrolyte, e.g., KOH.

The air gaps 180(1)-180(N) provide crystal lattice momentum for opticaldiffraction to advantageously result in increased external quantumefficiency similar to that achieved by the approach of having aplurality of protrusions repeated in a two-dimensional pattern on asurface of a C plane (0001) sapphire substrate. The air gaps180(1)-180(N) also aid heat dissipation from the LED structure duringoperation. However, the proposed technique of the present disclosureadvantageously removes the source of material contamination due to themask layer.

In one embodiment, the air gaps 180(1)-180(N) are filled with athermally conductive material, which may comprise a plurality ofnanoparticles, a plurality of compound particles, or a combinationthereof. Additionally or alternatively, the air gaps 180(1)-180(N) arefilled with an electrically conductive material, which may comprise aplurality of nanoparticles, a plurality of compound particles, or acombination thereof.

IIIustrative Suppression of Propagation of Screw Dislocations

FIGS. 2A-2C are diagrams showing progression in suppression ofpropagation of screw dislocations in an LED structure in accordance withan embodiment to the present disclosure.

FIG. 2A illustrates an LED structure having a substrate 210, a firstIII-nitride 220 deposited thereon, and a second III-nitride layer 230deposited on the first III-nitride layer 220. The first III-nitridelayer 220 has a low bandgap energy E_(g,Lo) and the second III-nitridelayer 230 has a high bandgap energy E_(g,Hi) where E_(g,Hi)>E_(g,Lo).V-notch grooves 240(1) and 240(2), commonly formed on the III-nitridesurface due to wet chemical etching in hot bases or acids such as KOH orH₃PO₄ to reveal the etch pit density, exist in the second III-nitridelayer 230 or in a combination of the second III-nitride layer 230 andthe first III-nitride layer 220, as shown in FIG. 2A. Screw dislocationsexist at the bottom of the V-notch grooves 240(1) and 240(2).

FIG. 2B illustrates that a surface passivation layer 250(1), 250(2) isformed in each of the V-notch grooves 240(1) and 240(2), respectively.The surface passivation layer 250(1), 250(2) selectively fills up thebottom portion of the respective V-notch groove 240(1), 240(2). In oneembodiment, the surface passivation layer 250(1), 250(2) comprises aIII-oxide layer. For example, when the second III-nitride layer 230 andthe first III-nitride layer 220 comprise In/GaN, the surface passivationlayer 250(1), 250(2) may comprise (In/Ga)₂O₃. In one embodiment, thesurface passivation layer 250(1), 250(2) is formed by selectivephoto-enhanced oxidation.

FIG. 2C illustrates that a third III-nitride layer 260 is formed on thesurface passivation layer 250(1), 250(2) and the second III-nitridelayer 230 by selective lateral epitaxy of the third III-nitride layer260 using an MOCVD process. Further layers, such as a layer 270, may beformed on or above the third III-nitride layer 260 as necessary.

Thus, by filling the V-notch grooves 240(1), 240(2) with the surfacepassivation layer 250(1), 250(2), the propagation of screw dislocationscan be suppressed. Unlike existing techniques, the proposed techniqueavoids the relatively higher costs and longer time associated with usingillumination alignment procedures. Issues such as undesirable impuritymigration and/or damage to the crystal atomic structure associated withSiO₂ masking or dry etch can also be avoided. The process can berepeated used for MOCVD regrowth in LED structures to achieve arelatively low density of dislocations, e.g., 10⁷-10⁶ cm⁻².

ILLUSTRATIVE EXAMPLES

FIGS. 3A-3B are photos of examples in accordance with an embodiment tothe present disclosure.

FIG. 3A shows an outer surface of [11 2 0]-axially grown III-nitridenanowires of GaN having been selectively transformed into a III-oxidelayer of Ga₂O₃ by selective photo-enhanced oxidation.

FIG. 3B shows a plurality of AlGaO stripes and a plurality of highbandgap AlGaN stripe openings over a low bandgap GaN/sapphire substrate.The AlGaO stripes are formed using selective photo-enhanced oxidation.

IIIustrative Processes

FIG. 4 is a flowchart diagram showing a process 400 of fabrication of anLED structure in accordance with an embodiment to the presentdisclosure.

At 402, the process 400 forms a first III-nitride layer with a first lowbandgap energy on a first surface of a substrate. For example, referringto FIG. 1A, the first III-nitride layer 120 is formed on the top surfaceof the substrate 110. At 404, the process 400 forms a second III-nitridelayer with a first high bandgap energy on the first III-nitride layer.For example, referring to FIG. 1A, the second III-nitride layer 130 isformed on the first III-nitride layer 120. At 406, the process 400transforms portions of the second III-nitride layer into a plurality ofIII-oxide stripes by photo-enhanced wet oxidation. For example,referring to FIG. 1B, portions of the second III-nitride layer 130 aretransformed into III-oxide strips 135(1)-135(M) by photo-enhanced wetoxidation. At 408, the process 400 forms a plurality of III-nitridenanowires with a second low bandgap energy on the second III-nitridelayer. For example, referring to FIG. 10, the III-nitride nanowires145(1)-145(N) are formed on the second III-nitride layer 130. At 410,the process 400 selectively transforms at least some of the III-nitridenanowires into III-oxide nanowires by selective photo-enhancedoxidation. For example, referring to FIG. 1D, the III-nitride nanowires145(1)-145(N) are transformed into III-oxide nanowires 145(1)-145(N) byselective photo-enhanced oxidation.

In one embodiment, the photo-enhanced wet oxidation may comprise a useof light source with photon energy exceeding the bandgap energy of saidIII-nitride and illuminate on a portion of said III-nitride whosesurface is not partially covered with masking materials such asphoto-resist. The wet oxidation process then takes place in anelectrolyte such as buffered CH₃COOH/CH₃COONH₄. In one embodiment, thephoto-enhanced wet oxidation may exercise on a III-nitride layeredstructure comprising a first top low bandgap and first bottom highbandgap III-nitride materials. More specifically, selectivephoto-enhanced wet oxidation with a photonic energy hy, whereE_(g,Lo)<hy<E_(g,Hi), can transform III-nitride compounds with E_(g,Lo)into III-oxide compounds. For example, III-nitrides such as low bandgapIn/GaN grown on GaN can be transformed into (In/Ga)₂O₃ on GaN byselective photo-enhanced wet oxidation.

In one embodiment, the III-nitride nanowires may comprise In/GaNnanowires, and wherein the III-oxide nanowires comprise (In/Ga)2O3nanowires.

In one embodiment, the substrate may comprise a C plane (0001) sapphiresubstrate, wherein the first III-nitride layer comprises a layer of GaN,and wherein the III-oxide stripes are aligned substantially parallel toa [11 2 0] or [1 1 00] direction of GaN.

In one embodiment, the method may further comprise: forming a thirdIII-nitride layer on the second III-nitride layer and over the III-oxidenanowires; and removing the III-oxide nanowires to form a plurality ofair gaps in the third III-nitride layer. For example, referring to FIGS.1E-1G, the third III-nitride layer 150 is formed on the secondIII-nitride layer 130 and over the III-oxide nanowires 145(1)-145(N),and the III-oxide nanowires 145(1)-145(N) are dissolved to make the airgaps 180(1)-180(N).

Forming a third III-nitride layer may comprise forming the thirdIII-nitride layer by lateral epitaxy of the III-nitride layer using ametal-organic chemical vapor deposition (MOCVD) process. Removing theIII-oxide nanowires may comprise dissolving the III-oxide nanowiresusing an acidic electrolyte, e.g., H₃PO₄, or a basic electrolyte, e.g.,KOH.

The method may further comprise filling the air gaps with a thermallyconductive material, the thermally conductive material comprising aplurality of nanoparticles, a plurality of compound particles, or acombination thereof. Alternatively or additionally, the method mayfurther comprise filling the air gaps with an electrically conductivematerial, the electrically conductive material comprising a plurality ofnanoparticles, a plurality of compound particles, or a combinationthereof.

In one embodiment, the method may further comprise forming a pluralityof III-nitride layers on the third III-nitride layer to form at leastone LED. For example, referring to FIGS. 1E and 1F, the III-nitridelayers 150, 160, 170 and 180 are formed to provide an LED structurehaving one or more LEDs therein.

FIG. 5 is a flowchart diagram showing a process 500 of fabrication of anLED structure in accordance with another embodiment to the presentdisclosure.

At 502, the process 500 forms a first III-nitride layer with a first lowbandgap energy on a first surface of a C plane (0001) sapphiresubstrate. At 504, the process 500 forms a second III-nitride layer witha first high bandgap energy on the first III-nitride layer. At 506, theprocess 500 forms an LED structure over the second III-nitride layer. At508, the process 500 forms, at least in part by selective photo-enhancedoxidation, in the LED structure a plurality of air gaps that areparallel to one another and adjacent the second III-nitride layer.Again, reference can be made to FIGS. 1A-1G as a result of theoperations of the process 500.

In one embodiment, forming, at least in part by selective photo-enhancedoxidation, a plurality of air gaps that are parallel to one another andadjacent the second III-nitride layer of a high bandgap energy maycomprise: transforming portions of the second III-nitride layer into aplurality of III-oxide stripes by photo-enhanced wet oxidation; forminga plurality of III-nitride nanowires with a second low bandgap energy onthe second III-nitride layer; selectively transforming at least some ofthe III-nitride nanowires into III-oxide nanowires by selectivephoto-enhanced oxidation; forming a third III-nitride layer on thesecond III-nitride layer and over the III-oxide nanowires; and removingthe III-oxide nanowires to form the plurality of air gaps.

In one embodiment, the photo-enhanced wet oxidation may comprisephoto-enhanced wet oxidation with a photon energy between the first highbandgap energy and the first low bandgap energy.

In one embodiment, removing the III-oxide nanowires may comprisedissolving the III-oxide nanowires using an acidic electrolyte or abasic electrolyte.

In one embodiment, the method may further comprise filling the air gapswith a thermally conductive material, the thermally conductive materialcomprising a plurality of nanoparticles, a plurality of compoundparticles, or a combination thereof.

In one embodiment, the method may further comprise, after forming thesecond III-nitride layer on the first III-nitride layer and beforeforming the LED structure over the second III-nitride layer, forming asurface passivation layer in a V-notch groove that is in the secondIII-nitride layer or in a combination of the second III-nitride layerand the first III-nitride layer by selective photo-enhanced oxidation.

In one embodiment, the surface passivation layer may comprise aIII-oxide layer.

In one embodiment, forming the LED structure over the second III-nitridelayer may comprise forming a third III-nitride layer on the surfacepassivation layer and the second III-nitride layer by lateral epitaxy ofthe III-nitride layer using an MOCVD process.

FIG. 6 is a flowchart diagram showing a process 600 of suppression ofscrew dislocations in an LED structure in accordance with an embodimentto the present disclosure.

At 602, the process 600 forms a first III-nitride layer with a first lowbandgap energy on a first surface of a substrate. At 604, the process600 forms a second III-nitride layer with a first high bandgap energy onthe first III-nitride layer. At 606, the process 600 forms a surfacepassivation layer of III-oxide in a V-notch groove that is in the secondIII-nitride layer or in a combination of the second III-nitride layerand the first III-nitride layer by selective photo-enhanced oxidation.Reference can be made to FIGS. 2A-2B.

In one embodiment, the method may further comprise forming a thirdIII-nitride layer on the surface passivation layer and the secondIII-nitride layer by lateral epitaxy of the III-nitride layer using anMOCVD process. Reference can be made to FIG. 2C.

Conclusion

The above-described techniques pertain to techniques of controllingoperations of LED lighting by using the output of a TRIAC dimmer not asa power source but as a lighting control signal. Although the techniqueshave been described in language specific to structural features and/ormethodological acts, it is to be understood that the appended claims arenot necessarily limited to the specific features or acts described.Rather, the specific features and acts are disclosed as exemplary formsof implementing such techniques.

It is appreciated that the illustrated apparatus 100 and apparatus 200are each one example of a suitable implementation of the proposedtechnique and is not intended to suggest any limitation as to the scopeof use or functionality of the various embodiments described. Anyvariation of the disclosed embodiments made by a person of ordinaryskill in the art shall be deemed to be within the spirit of the presentdisclosure, and thus shall be covered by the scope of the presentdisclosure.

What is claimed is:
 1. A structure comprising: a substrate structure; afirst III-nitride layer with a first low bandgap energy formed on asurface of the substrate; a second III-nitride layer with a first highbandgap energy formed on the first III-nitride layer; a plurality ofIII-oxide stripes formed on the second III-nitride layer; and aplurality of III-oxide nanowires formed on the second III-nitride layerbetween the III-oxide stripes.
 2. The structure as recited in claim 1,wherein at least some of III-nitride nanowires with a second low bandgapenergy formed on the second III-nitride layer between the III-oxidestripes are selectively transformed into the III-oxide nanowires byselective photo-enhanced oxidation.
 3. The structure as recited in claim2, wherein the selective photo-enhanced oxidation comprisesphoto-enhanced wet oxidation with a photonic energy between the firsthigh bandgap energy and the first low bandgap energy.
 4. The structureas recited in claim 2, wherein the III-nitride nanowires comprise In/GaNnanowires, and wherein the III-oxide nanowires comprise (In/Ga)₂O₃nanowires.
 5. The structure as recited in claim 1, wherein the pluralityof III-oxide straps are formed by transforming portions of the secondIII-nitride layer by photo-enhanced oxidation.
 6. The structure asrecited in claim 1, wherein the substrate comprises a C plane (0001)sapphire substrate, wherein the second III-nitride layer comprises alayer of GaN, and wherein the III-oxide stripes are alignedsubstantially parallel to a [11 2 0] or [1 1 00] direction of GaN. 7.The structure as recited in claim 1, further comprising: a thirdIII-nitride layer formed on the III-oxide straps and the III-oxidenanowires.
 8. The structure as recited in claim 7, further comprising: aplurality of III-nitride layers formed on the third III-nitride layer toform at least one light-emitting diode (LED).
 9. A structure comprising:a substrate structure; a plurality of III-oxide stripes formed on afirst surface of the substrate structure; a plurality of III-oxidenanowires formed on the first surface of the substrate structure betweenthe III-oxide stripes; and a light-emitting diode (LED) structure formedover the III-oxide straps and the III-oxide nanowires.
 10. The structureas recited in claim 9, wherein at least some of III-nitride nanowiresformed on the first surface of the substrate structure between theIII-oxide stripes are treated with selective photo-enhanced oxidation toform the plurality of III-oxide nanowires.
 11. The structure as recitedin claim 9, wherein the substrate structure comprises: a substrate; afirst III-nitride layer with a first low bandgap energy formed on asurface of the substrate; and a second III-nitride layer with a firsthigh bandgap energy formed on the first III-nitride layer; wherein asurface of second III-nitride layer is the first surface of thesubstrate structure.
 12. The structure as recited in claim 11, whereinthe plurality of III-oxide straps are formed by transforming portions ofthe second III-nitride layer by photo-enhanced oxidation.
 13. Thestructure as recited in claim 12, wherein the selective photo-enhancedoxidation comprises photo-enhanced wet oxidation with a photonic energybetween the first high bandgap energy and the first low bandgap energy.14. The structure as recited in claim 9, wherein the III-oxide nanowiresare capable of being removed to form a plurality of air gaps.
 15. Thestructure as recited in claim 14, further comprising: a thermallyconductive material filled in the air gaps, and the thermally conductivematerial comprising a plurality of nanoparticles, a plurality ofcompound particles, or a combination thereof.
 16. The structure asrecited in claim 14, further comprising: an electrically conductivematerial filled in the air gaps, and the electrically conductivematerial comprising a plurality of nanoparticles, a plurality ofcompound particles, or a combination thereof.
 17. A structurecomprising: a substrate; a first III-nitride layer with a first lowbandgap energy formed on a first surface of a substrate; a secondIII-nitride layer with a first high bandgap energy formed on the firstIII-nitride layer; a plurality of V-notch grooves formed in the secondIII-nitride layer or in a combination of the second III-nitride layerand the first III-nitride layer; and a III-oxide layer formed in theV-notch groove.
 18. The structure as recited in claim 17, furthercomprising: a light-emitting diode (LED) structure formed over theIII-oxide layer and the I second III-nitride layer.